Apparatus for mixing flowable materials



United States Patent Inventor 3,251,581 5/1966 Jensen 259/108 55CMMMMIIMMNW my 3,330,538 7/1967 Gableretal. 259/107 3,358,971 12/1967Steinbook,.lr.. 259/107 [2 pp 713,523 3,385,568 5/1968 Gray 259/10sxF119! 1511963 3,161,404 12/1964 Jay 259/134 [45] Patented Dec. 1, 1970Primary Examiner-Edward l... Roberts [54] APPARATUS FOR MIXING FLOWABLEMATERIALS ABSTRACT: An a aratus for mixin flowable materials in chims'zsDnwhgnp' the form of pow/deg, flake, granules, 5nd of fluids in a wide[52] US. Cl. 259/107, range f viscosities, has in a container 3 framerotating about a 259/134 vertical axis. The frame comprises impellersfor radial movem 7/ ment of the material at the top and at the bottom ofthe con- Search miner and members in between for vertical movement Flow0 Cream Haze van), patterns of closed loops in vertical planes areenforced for batch mixing by rotating the frame in one direction, and[56] Cm reversed flow patterns for continuousmixing by reversing theUNITED STATES PATENTS frame rotation, as well as for complete discharge.The impel- 2,269,301 l/ 1942 Anstice 259/107 lers and the members carryvanes oblique to the mixer ele- 2,272,7l5 2/1942 Lindsey ..259/(l.C.FVert.)UX menis- Patented Dec. 1, 1970 Sheet 1 0:3

Y Z 6 F FIG.4

mm A mH vK mC E 4 A 4 m 4 u a w\ n Z 4 5 4 1 :L a. w EMF. Emu I\ A. w wFo IN 4 Patented, Dec. 1, 1970 3,544,081

Sheet 3 023 INVENTOR. HANS A. ECKHARDT Patented Dec. 1, 1970 Y 3,544,081

Sheet 3 o! 3 3% B FIG.9C 4. J22 4 INVENTOR. HANS A. ECKHARDT 1 APPARATUSFOR MIXING FLOWABLE MATERIALS The present invention relates to the fieldof mixing, and more particularly, to apparatus for mixing flowablematerials which include particulate materials in the form of powder,flake, granules, and fluidmaterials in a wide range of viscosities.Combined with mixing are often such operations as agitating, stirring,dispersing, shearing, kneading, contacting, drying, devolatilizing,reacting, discharging and wiping of inner surfaces of the apparatus. Forthe sake of simplicity, for any of such combined operations the termmixing will be used in this specification. q

In the prior art a great number of mixing devices is known, each oneusually designed for specific mixing purposes and generally not suitableto solve other mixing problems. Among the vertical mixers there aredevices known wherein blades or ribbons are applied which, however, failto guide the flowable material in a closed loop of flow in radialdirections at the top and at the bottom of the mixer vessel, and invertical directions therebetween.

Vertical mixers as well as horizontal mixers are known wherein helicalribbons are arranged to move the material coaxially to the mixer axis,in the case of vertical mixers vertically. It has been found, however,that particles, including larger and harder ones, are conveyed by theblades which wipe the sidewalls of the receptacle, all the way to oneend of the mixer, usually the bottom, where they remain without beingintermixedwith other material portions and without being effectivelysheared or comminuted, lacking the necessary shearing forces in thatlocation. As a whole, the material carried by the helical ribbon alongthe receptacle surface receives little intermixing with other materialportions.

Other material particles -and especially foreign objects, frequently ofmetal, stone or ceramics, get sheared and squeezed by, the long helicalribbon along the receptacle wall from where no release or escapeis'offered. They cause metal abrasion on the mixer partsand undesirablecolor development in the case of transparent-or translucent products.With explosives such as solid propellants the consequences of suchintensive local shear connected, with heat generation have beendisastrous. In addition, a helical ribbon which is to wipe the innersurface of the receptacle with close clearance is expensive to make anddifficult to maintain, especially in high shear applications. If thehelical ribbons are made sufficiently rigid and heavy, a relativelylarge portion of the mixer volume is taken up by these mixing elements.

The present invention contemplates an improvement in apparatus formixing a wide variety of flowable materials, such as finely dividedsolids, for example, in powdery, flaky, granular form or liquids fromlow to high viscosities.

It is an object of this invention to provide an improved, more effectiveand at the same time simpler, less expensive mixing apparatus which iseasy to clean and to maintain.

It is a further object to provide a mixing apparatus enforcing flowpatterns in form of closed loops by moving the material radially at thetop and at the bottom of the apparatus, and to move it vertically alongthe inner surface of the apparatus as well as in the center, however, inopposite'directions.

It is another object to provide a mixing apparatus suitable forcontinuously discharging and replenished hoppers wherein a closed loopflow is established by forcing the material at the top of the apparatusradially outward, along the sidewalls of the apparatus downward, at thebottom of the apparatus radially inward, and at the center of'theapparatus upward.

It is a further object to provide a mixing apparatus especially suitablefor certain types of flowable materials wherein a closed loop flow isestablished by forcing the material at the bottom of the apparatusradially outward, along the sidewalls of the apparatus upward, atthe topof the apparatus inward and in the center of the apparatus downward.

It is another object to provide a mixing apparatus which establishes aclosed loop flow in one direction'advantageous during the mixing periodof a batch mixing cycle, and which establishes a closedloop flow in theopposite direction by reversing the direction of rotation of the mixingelement, advantageous during the discharge period of the batch mixingcycle.

It is a further object to provide a mixing apparatus which breaks upstrata of materials by interspersing portions of strata into portions ofother strata in vertical and radial directions.

It is a further object to provide a mixing apparatus which subjectsparticles, especially harder and larger ones, intermittently toshearingforces in individual areas which are succeeded by other areas providedfor releasing, interspersing and intermixing such particles into othermaterial portions, before they are subjected again, to shearing forcesin other individual areas. i

It is a further object to provide a mixing apparatus which rejects andreleases foreign objects from shearing forces thus avoiding metalabrasion and frictional heat generation, and which permits the releaseand rejection of larger foreign objects by resilient opening andcontraction of the passages for such release and rejection.

, It is another object to provide a mixing apparatus which has an innersurface completely wiped by mixing elements with close clearance.

It is another object to provide a mixing apparatus with mixing elementswhich resist distortion and bending, which at the same timeare of lightweight and of small volume, and which are simple to manufacture and tomaintain.

These and other objects, novel features and advantages will be moreapparent from the following description taken in connection with theaccompanying drawings wherein:

FIG. 1 is a perspective view of one embodiment with parts broken away;

FIG. 2 is an elevation of the same embodiment, partly in section;

FIG. 3 is a perspective view of another embodiment, with parts brokenaway;

FIG. 4 is an elevation of an embodiment similar to that of P16. 3;

' FIGS. 'SA, 513, and 5C shows plan and cross-sectional views ofelements from FIGS. 1, 2, 3, 4;

FIGS. 6A, 6B and 6C, FIGS. 7A, 7B, and 7C, FIGS. 8A, 8B, and 8C, FIGS.9A, 9B, and 9C, FIGS. 10A, 10B, and 10C, FIGS. 11A, 11B, and 11C, andFIGS. 12A, 12B, and 12C are plan and cross-sectional views of otherelements which may be used with the apparatus of the invention.

Referring to the drawings, FIGS. 1 and 2, there is shown an apparatusfor mixing flowable materials comprising a container 12 having asubstantially vertical axis and inner surfaces 14, 15 and 16 symmetricto this axis, a feed opening 18 and a discharge opening 19, both shownas being closed. A frame 20 is mounted for rotation by the shaft 21which has an axis coinciding with the axis of the container 12, and bydrive means (not shown). The frame 20 comprises a top impeller 22 whichextends in the upper part of the container 12 from the shaft 21 outwardtoward the inner surfaces 15 and which is shaped to move material in aninward direction having a component radial of the shaft 21, and to wipethe inner surface 14 of the container 12 with close clearance.

The frame 20 includes the outer member 23 which extends from the outwardendof the top impeller 22 downward along the inner surface 15 and whichis shaped to wipe the inner surface 15 with close clearance, and to movematerial in an upward direction having a vertical component.

The frame 20 also comprises a bottom impeller 24 which extends in thelower part of the container 12 from the axis outward, and is connectedto the lower end of the outer member 23. The bottom impeller 24 isshaped to wipe the inner surfaces 16 with close clearance and to movethe material in an outward direction having a component radial of saidaxis.

The frame 20 further includes an inner member 25 which extends from thetop impeller '22 to the bottom impeller 24 and which has a smallerradial distancefrom said axis at its upper end than at its lower end.The inner member 25 is shaped to move the material in an upwarddirection having a The top impeller 22, the outer member 23 and thebottom impeller 24 comprise as a rigid profile a strip 26, and along theperiphery thereof toward the inner surfaces l4, 15, 16 a plurality ofvanes 27. The edges on one side of the vanes 27 are connected to therigid strip 26, and the edges on the opposite side are oblique and inclose proximity to the inner surfaces 14, 15, 16, the vanes 27 beingdisposed oblique to the strip 26. The inner member 25 consists of anelongated rigid strip 28 to move through the material with its front rim28' first, and a plurality of vanes 29, each vane connected on the edge29 to a flat side of the strip 28, the vanes 29 being disposed obliqueto the strip 28.

In operation, when the shaft 21 is rotated in clockwise direction asseen in a plan view, as indicated, the bottom impeller 24 moves theflowable material from the center outward, the outer member 23 and theinner member 25 move the material upward and the top impeller 22 movesthe materi al inward which then moves downward in the center by gravity.The downward movement in the center is improved by the smaller distanceof the inner member 25 from the axis at the top impeller 22 than atthebottom impeller 24. The inner member 25 cuts out a conical-body ofmaterial which sinks down by gravity toward the bottom where it issliced into thin layers and plowed to the outside by the bottom impeller24. A closed loop flow of the material in vertical planes through theaxis is thus achieved.

The vanes 27 wipe the inner surfaces 14, 15, 16 with close clearance.Undispersed particles are subjected to shearing forces between thewiping edges of the vanes 27 and the surfaces 14, 15, 16 and, afterhaving passed the vanes 27, are interspersed into other materialportions, until they receive subsequent shear work by other vanes 27.Foreign objects, after having travelled along a vane 27, are thenrejected into other material portions. In many instances, for example,for complete discharge, it is advantageous to reverse the rotation ofthe shaft 21; the outer member 23 will then wipe the material downwardand the bottom impeller 24 inward and downward.

FIG. 2 shows the same embodiment, with the frame 20 in the diametralposition indicated in dotted lines. The flow directions of the materialadvantageous for the first phase of, the mixing cycle are indicated withsolid arrows, the flow directions in the second-phase, for completedischarge, with the direction of rotation reversed, with dotted arrows.

FIG. 3 illustrates another embodiment particularly suitable forcontinuous mixing in a hopper or container 32 with con: tinuousdischarge through a discharge opening 39 in the bot-, tom surface 36,shown as being open, into a discharge duct 37, and continuous orintermittentfeeding through a top opening 38, shown as being open. Theother inner surfaces are designated with 35.

The frame 40 is mounted for rotation by the shaft 41 and by drive means(not shown) around an axis coinciding with the axis of the container 32.A top impeller 42 consists-of the long top impeller arms 42' definingthe top impeller perimeter, and the short top impeller arms 41", bothextending in the upper part of the container 32 from the shaft 41outward toward the inner surface 35 and shaped to move the materialoutward, when rotated in clockwise direction as seen in a plan view, asindicated. From the outer ends of the long top impeller arms 42, theouter members 43 extend downward aLong the inner surface 35 shaPed towipe the inner surface 35 with close clearance and to move the materialdownward.

In the lowest part of the container 32, a bottom impeller 44 consists ofthe long bottom impeller arms 44' defining the bottom impellerperimeter, and the short bottom impeller arms 44", which extend from theaxis outward and are shaped to move the material outward. The longbottom impeller arms 44 connect to the lower ends of the outer members43; the short bottom impeller arms 44' connect to the inner members 45which extend to the ends of the short top impeller arms 42", and areshaped to move the material upward. The lower ends of the inner members45 have a smaller radial distance from the axis than the upper ends.

The outer members 43 consist of rigid profiles, in this case of strips46 and along their periphery toward the surface 35 a plurality of vanes47, their edges on one side being connected to the strips 46, theiredges on the opposite side being oblique and in close proximity tothe-inner surface 35, and oblique to the strips 46.

The inner members 45, the top impeller 42 and the bottom impeller 44consist of rigid strips 48 to move through the material with their frontrims 48' first, and a plurality of vanes 49, each vane connected on oneedge 49 to a flat side of the strip 48 and disposed oblique to thatstrip 48.

In operation, when the shaft 41 is rotated in clockwise direction, asseen in a plan view and as indicated, the top impeller 42 moves theflowable material as it enters through the top opening 38 outward, theouter member 43 moves the material downward along the inner surface 35.The material is moved inward at the bottom by the bottom impeller 44,and upward in the center by the inner members 45. The upward movement inthe center is improved by a smaller radial distance of the inner member45 from the axis at the bottom than at the top of the container 32,essentially decreasing the tendency of the material to move downward inthe center.

Closed loop flow patterns of the material in vertical planes through theaxis are thus achieved, which has been found to be advantageous for mostmixing operations. Many materials tend to flush and rathole through thecenter, leaving the peripheries toward the inner surface 35 more or lessstagnant, and this drawback has been eliminated by this development.

FIG. 5 shows a mixing and wiping element similar to that used in FIGS.1, 2, 3, 4 in more detail. A is a plan view, B is a section along thelines B-B of A, C is a section along lines C-C of A. This element forwiping a surface with close clearance and for transversely moving anddiverting flowable material comprises as a profile a rigid strip 56 andalong the periphery thereof toward the surface 55 a plurality of vanes57, their edges on one side being connected to the strip 56 and theiredges on the opposite side oblique and in close proximity to the surface55, the vanes 57'being disposed oblique to the strip 56. As shown at theright side of A and B, the vanes 57 overlap as projected in thedirection of their movement indicated by a solid arrow, so that thevanes 57 wipe the surface 55 completely. The flow of material isindicated by dotted arrows, in this as well as in all subsequentfigures.

At the left side of A and B, the element is shown as produced from anelongated L-shaped profile 50 comprising the rigid strip 56' and thestrip-shaped ribbon 51 having one edge, the wiping edge 52, in closeproximity .to the surface 55, by machining from the wiping edge 52 aplurality of slots 53 in directions transverse and parallel to thecenter line of the profile 50. By bending the portions of the ribbon 51between the slots 53 out of the plane of the ribbon 51, the vanes 57 areformed.

The vanes 57 and 57' are constructed to be resilient by using resilientmaterial and by having only part of their edges adjacent to the rigidstrips 56,56 attached thereto, for example by welding or soldering onlythe right portions of the vanes 57;57' to the strips 56;56' and bymachining the slots 53 between the vanes 57' and the rigid strip 56'sufficiently far to the right. As an essential feature, the vanes57;57'are thus constructed'and attached so that they bend under forces imposedby the material toward the direction of such forces, as indicated by thevane 57" in dotted lines, and after decrease of such forces resile backtoward their initial position.

FIG. 6 shows a different element for wiping a surface and transverselymoving material. A is a plan view, B is a section along the lines B-B ofA, C is a section along the lines C-C of A. The element is produced froma longitudinal L-shaped profile 60 which comprises the strip-shapedribbon 61 having one edge, the wiping edge 62, in close proximity to thesurface 65, by machining from the wiping edge 62 a plurality of slots63, and by bending the portions 67 and 67 of the ribbon 61 between theslots 63 out of the plane of the ribbon 61 thus forming the vanes 67.The vane portions 67" are attached to the rigid strip 66 by welding orsoldering, while the vane portions 67 are resilient.

FIG. 7 illustrates another element for wiping a surface and transverselymoving material. A is a plan view, B is an elevational view along thelines B-B of A, C is a section along the lines CC of A. The element isproduced from a T-shaped profile 70 comprising two strip-shaped ribbons71,71 by machining from the wiping edge 72 and the edge 72' a pluralityof slots 73,73 and by bending portions of the ribbons 71,71 between theslots 73 on one side out of the plane of the ribbons 71,71 thus formingthe vanes 77,77. In order to have the wiping edges 72 extending in closeproximity to the surface 75, the wiping edges 72 are machined to runparallel and concentric to the surface 75.

FIG. 8 shows a different element for wiping a surface and transverselymoving material. A is a plan view, B is an elevation in the directionB-Bof A, C is a s section along the lines C-C of A. The element is producedfrom a T-shaped profile 80 comprising two strip-shaped ribbons 81, 81'by machining from the wiping edge 82 and the edge 82' a plurality ofslots 83, 83' and by bending portions of the ribbons 81, 81' between theslots 83, 83 on opposite sides in opposed directions out of the plane ofthe ribbons 81, 81 thus forming the vanes 86, 86'. To have the wipingedges 82 extending in close proximity to the surface 85, the wipingedges 82 are machined to run parallel and concentric to the surface 85.

It is apparent that the elements shown in FIGS. 7 and 8 have greaterresistance to bending, torque and twist than the elements of the FIGS. 5and 6 and it is practical to improve their mechanical resistance and thecapacity to mix and to move material transversely by providing vanes 57,67 on the opposite sides of the rigid strips 56, 66. Advantageously, forincreased mechanical stability, the vanes on one side of the rigid stripshould be staggered in relation to the vanes on the op posite side ofthat strip, as FIGS. 7 and 8 show.

It is also to be noted that of the wiping elements shown, only those inthe FIGS. 5, right side, and 9 wipe a surface completely and arerecommended for use in a mixer as shown in FIGS. 1 and 2, where only oneouter member 23 wipes the inner surface 15. In this mixer, wipingelements as in FIGS. 5, left side, 6, 7, 8 would leave stripes of thesurface unwiped and they are therefore suitable for a mixer as shown inFIGS. 3 and 4 wherein the surface 35 is wiped by two outer members 43.It is understood that the vanes 47 along one outer member 43 arestaggered in relation to the vanes 47 along the diametral outer member43.

FIG. 9 illustrates a further element for wiping a surface andtransversely moving material. A is a plan view, B is a section along thelines 13-13 of A, and C is a section along the lines C-C of A. Theelement comprises a rigid strip 90 which is oblique to the surface 95and has along its periphery toward the surface 95 a plurality of vanes97, their edges 91 on one side being connected to the rigid strip 90,and the edges along the opposite side, the wiping edges 92, oblique andclose proximity to the surface 95, the vanes being disposed oblique tothe rigid strip 90. The vanes 97 overlap as projected in the directionof movement, so that they wipe the surface 95 completely.

In the FIGS. 5, 6, 7, 8, 9 elements for wiping a surface andtransversely moving material have been presented, suitable for the topimpeller 22, the outer member 23 and the bottom impeller 24 in FIG. 1,and for the outer members 43 in FIG. 2. For the same applications theelements described in the following FIGS. 10, ll, 12 can be successfullyused,'although they have been developed particularly for transverselymoving material without wiping a surface, like the inner member 25 inFIGS. 1 and 2, the top impeller 2, the bottom impeller 44, and the innermembers 45 in FIGS. 3 and 4.

FIG. 10 shows an elongated element for travelling through flowablematerial transversely to its center line, for moving material in thedirection of that center line. A is a plan view of this element, B is asection along the lines 13-3 of A, and C is a section along the linesC-C of B. The element comprises the two rigid strips 102, 103 movingthrough the material with the front rims 102', l03'first, and aplurality of vanes 105, each vane connected on one edge 106 to a flatside of the strip 102, and connected on the opposite edge 107 to a flatside of the strip 103, said vanes being disposed oblique to the strips102, 103. The element is shown at the left side as produced from aU-shaped profile.

FIG. 11 illustrates another element for moving material transversely toits center line. A is a plan view, B is a section along the lines B-B inA, and C is a section along the lines C-C in B. The element comprisestwo rigid strips 112, 113 moving through the material with their frontrims 1121.113 first, and a plurality of vanes 115, each vane connectedon one edge 116 to the flat side of the strip 1 12, and connected on theopposite edge 117 to a flat side of the strip 113, with portions of thevanes being disposed oblique to the strips 112, 113. The element isshown as produced from an H-shaped profile.

FIG. 12 pictures a further element for moving material transversely toits center line. A is a plan view, B is a section along the lines B-B ofA, and C is a section along the lines C-C of B. The element comprisestwo oblique rigid strips 122, 123 moving through the material with theirfront rims 122' 123' first, and a plurality of vanes 125, each vaneconnected on one edge 126 to an oblique flat side of the strip 122, andon the opposite edge 127 to an oblique flat side of the strip 123, thevanes being disposed oblique to the element. The vanes 125 may be flushwith the front rirns 122', 123 or may remain below, or may extendbeyond, as shown.

As described in connection with the drawings, a versatile mixer suitablefor batch mixing as well as for continuous mixing, both with completedischarge, is thus obtained. The change from batch to continuousoperation and vice versa usually requires merely to reverse the rotationof the mixer elements. Furthermore, a mixer as described in thisspecification offers a considerable reduction in mixing time and anessential improvement of product quality.

While the present invention has been described in connection withpreferred embodiments in an illustrative and not in a limiting sense, itis apparent that modifications and variations may be resorted to withoutdeparting from the spirit and the scope of the invention as thoseskilled in the art will readily understand. Such modifications andvariations are considered to be within the purview and scope of theinvention and appended claims.

Iclaim:

1. Apparatus for mixing flowable materials comprising:

a container for receiving and containing material, having asubstantially vertical axis and inner surfaces symmetric to said axis;

a frame in said container mounted for rotation around an axis coincidingwith said axis of said container, said frame comprising:

a top impeller extending in the upper part of said container from saidaxis outward toward inner surfaces of said container and shaped to movesaid material in a direction having a component radial of said axis;

' at least one outer member extending from the outward perimeter of saidtop impeller downward along inner surfaces of said container, shaped towipe said inner surfaces with close clearance, and to move said materialin a direction having a vertical component.

2. Apparatus for mixing flowable materials as described in claim 1,wherein a bottom impeller extends in the lower part of said containerfrom said axis outward and connects to the lower end of said outermember, said bottom impeller being shaped to wipe inner surfaces withclose clearance. and shaped to. move said material in a direction havinga component radial of said axis.

3. Apparatus for mixing flowable materials as claimed in claim 2,wherein at least one inner member extends from a within the perimenterof said bottom impeller.

4. Apparatus for mixing flowable materials as claimed in claim 3,wherein said innei member is shaped to move said material in a directionhaving a vertical component.

5. Apparatus for mixing flowable materials as claimed in claim 3,wherein the upper end of said inner member has a smaller radial distancefrom said axis than the lower end of said inner member.

6. Apparatus for mixing flowable materials as claimed in claim 3,wherein the lower end of said inner member has a smaller radial distancefrom said axis than the upper end of said inner member.

7. Apparatus for mixing flowable materials as claimed in claim 1,wherein the top impeller is shaped to wipe inner surfaces of saidcontainer with close clearance.

8. Apparatus for mixing flowable materials as claimed in claim whereinsaid top impeller is shaped to move material radially outward, saidouter member shaped to move the material vertically downward, and saidbottom impeller shaped to move the material radially inward, with saidframe rotated in one direction, whereas, with said frame rotated in 2 arigid profile and along the periphery thereof toward said surfaces aplurality of vanes, their edges on one side being connected to saidprofile, their edges on the opposite side being oblique and in closeproximity to said inner surfaces, said vanes being disposed oblique tosaid profile, and wherein said inner member shaped to LII move saidmaterial vertically, and said top impeller shaped to move said materialradially, comprise at least one rigid strip to move through saidmaterial with its front rim first, and a plurality of vanes, eachconnected on one edge to a flat side of said strip, said vanes beingdisposed oblique to said profile. v .1 a: 10. An element for wiping asurface with close clearance and for transversely moving flowablematerial comprising a T- shaped profile consisting of a base rib and twoside ribbons, each side ribbon having along the periphery thereof aplurality of vanes, their edges on one side being connected to saidprofile, and. their edges on the opposite side oblique and extending inone side ribbon towardsaid surface in close proximity to said surface,said vanes being disposed oblique to said profile.

11. Apparatus for mixing flowable materials as claimed in claim 9wherein said vanes overlap as projected in the direction of movement, sothat they wipe said surface completely.

32. Apparatus for mixing flowable materials as claimed in claim 9,wherein said member is produced from an elongated profile comprising atleast one strip-shaped ribbon having one edge, the wiping edge, in closeproximity to said surface, by machining a plurality of slots extendingfrom said wiping edge into said ribbon, and by bending portions of theribbon between said slots out of plane of said ribbon thus forming saidvanes.

13. Apparatus for mixing flowable materials as claimed in claim 12,wherein said slots extend into the ribbon from said wiping edge indirections transverse and parallel to the center line of said elongatedprofile.

14. Apparatus for mixing flowable materials as claimed in claim 13,wherein the vanes are constructedto be resilient and to bend underforces imposed by said material toward the direction of said forces, andafter a decrease of saidforces to resile back toward their initialposition.

